Anti-jamming apparatus

ABSTRACT

A linearly polarized wanted signal is separated from a nonlinearly polarized unwanted signal by transforming the wanted signal to a non-linearly polarized signal and the unwanted signal to a linearly polarized signal and transmitting these signals to a polarized filter to pass only t he component of the wanted signal orthogonal to the unwanted signal.

United States Patent 1191 Bobrow et al.

[ 1 3,733,607 51 May 15, 1973 s41 ANTl-JAMMING APPARATUS 2,881,432 4 1959 l-latkin ..343/756 [75] Inventors: Edwin N. Bobrow, Palos Verdes 2,952,017 9/1960 Easy Estams; Lawrence Murdock, 3,025,513 3/1962 Easy ..343/100.3 Torrance bmh Calif- FOREIGN PATENTS OR APPLICATIONS {73] Assignee: Thompson Ramo Wooldridge, Inc., 750,062 6/1956 Great Britain ..343/756 Cleveland, Ohio 1,099,996 3/1955 France ..343/756 [22] Wed: 1958 Primary Examiner-Samuel Feinberg U PP 766,171 Assistant Examiner-Richard E. Berger Attorney-Daniel I. Anderson, Donald R. Nyhagen 52 us. (:1. ..343/10o PE, 325 473, 333/21 A, and Edwm 0W 343/18 E 511 1m. 01 ..G0ls 7/36 [571 ABSTRACT [58] Field of Search ..343/756.5, 100.3, A linearly polarized wanted signal i separated from a 343/100 PE; 333/21 A124 61 211 21 A; non-linearly polarized unwanted signal by transform- 325/473 ing the wanted signal to a non-linearly polarized signal and the unwanted signal to a linearly polarized signal [56] References C'ted and transmitting these signals to a polarized filter to UNITED STATES PATENTS pass only t he component of the wanted signal 2,881,398 4/1959 Jones ..333/21 wanted Signal- 13 Claims, 6 Drawing Figures /l4 LOAD i QQ 'EA POLARlZATION PHASE. pOLARlZATlON RADAR RorAToR SHlFTER SEPARATOR RECEIVER ANTENNA Patented May 15, 1973 3,733,607

2 Sheets-Sheet 1 liq. I

/4 LOAD [I0 [I I /\Z 5 5 QQEAQ pOLARlZATlON PH ASE. POLARQATION RADAR ANTENNA RoTAToR 5m FTER SEPARATOR REQEWER EDWIN A/. BOB/20W LAWREN CL, MURDOCK QM M I VENT Q85 Patented May 15, 1973 2. Sheet N 508 20 W /N 55MZ2- C. MURDOCK ANTI-JAMMING APPARATUS The present invention relates to anti-jamming apparatus and more particularly to a method and apparatus employing diversity polarization reception as a technique for reducing jamming efiects.

Systems, such as radars, which depend for their operation on collecting electromagnetic energy from space and processing it may be denied this information because of jamming which, in its many forms, serves to increase the entropy of the received energy. It is well known, for example, that the operational capability of present-day radar systems is seriously degraded by electronic countermeasures of which jamming is one example.

Early efforts toward combatting jamming were directed principally toward preventing receiving system overload and led to black boxes of the Fast Time Constant, Detector Balance Bias, Sensitivity Time Control and Antenna Video Noise Leveler. However, as jammers became more sophisticated, other techniques such as frequency diversity, correlation detection, monopulse tracking and pulse doppler were developed which depend for their operation on exploiting either a particular strength of their own or a particular weakness of a jammer.

One weakness of a jammer which so far has escaped effective exploitation is the fact that to be effective against a general radar complex, that is, a coordinated group of radar systems, the jammer must radiate energy at several or all polarizations. Stated somewhat differently, since a particular radar system generally uses only one polarization, jamming systems must possess an operational capability against radar systems of varying polarization characteristics. However, due to the complexity of equipment necessary to quasiinstantaneously determine and retransmit a radar signal with the proper frequency and polarization characteristics, jamming devices in general utilize a simple type of polarized transmission such as slant-linear, or more usually, elliptical. Consequently, the transmitted jamming signal does not possess the same polarization characteristics as that of the target echo which in general will show large variations of the complex reflection coefficient with the polarization of the incident radar signal, and this knowledge can be used to combat jamming effectiveness.

It is, therefore, an object of the present invention to provide a method and'apparatus employing the technique of diversity polarization for reducing jamming effects.

It is another object of the present invention to provide a method and apparatus for effectively separating information signals from jamming signals by discriminating between the different types of polarizations associated with these signals.

The present invention provides a very effective means for discriminating against jamming as well as other unwanted signals, thereby substantially reducing their interfering effects, by a novel technique for processing the received signals according to their polarizations. According to the basic concept of the present invention, the polarizations of jamming signals are reduced to a linear form of polarization which is spatially out of phase with the polarization of the desired information signal. This angular difference between the two polarizations makes it possible for one to be separated from the other.

ming energy being achieved by means of a one-mode coupler device. The desired directions of polarization for the target and jamming signals are obtained either by suitably rotating the waveguide through which the respective signal energies are propagated or by means of a device such as a ferrite Faraday rotator.

Of course, any target reflection component with the same linear polarization as the jamming signal is lost along with the unwanted jamming signal. However, an-

alytical and laboratory studies indicate that signal to jamming enchancement of greater than 30 db. may be obtained which, interpreted in terms of radar range, means an increase in radar range over 30 times against a jamming target. No known technique is at present able to provide this signal enhancement without very great complexity in the equipment.

In addition to their anti-jamming usefulness, the method and apparatus of the present invention'can be used in many other applications as well where the ability to operate on the polarization characteristics of received signals is a desirable feature. It can, for instance, distinguish between true radar targets and false targets produced by a repeater with multiple polarization characteristics. It can be used to enhance a signal against a background of clutter if the polarization characteristics of the signal energy differ from those of the background. Also, the operational capacity of communication systems may be very greatly increased with the aid of the present invention due to thefact that information in at least two polarizations may be transmitted in the same channel and later be separated outfrom each other.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

FIG. 1 is a block diagram of one anti-jamming apparatus according to the present invention; and

FIGS. 2a-2d illustrate the successive polarizations of the information and jamming signals from the time they are received until they are separated out; and

FIG. 3 is a drawing, partly in cross-section, of aspecific embodiment of the block diagram of FIG. 1.

Referring now to the drawings, there is shown in FIG. 1 an anti-jamming system according to the present invention and, as shown therein, the system comprises first an antenna 10 adapted for diversity polarization reception. More specifically, antenna 10 is adapted to receive signals regardless of their, polarization and for this reason is referred to as an omnipolarization antenna. By way of example, a circular horn may be used as such an antenna.

A device defined as a polarization rotator 11 which is capable of rotation to the direction of polarization of signals applied thereto is connected between antenna and a phase shifter element 12. Rotator 11 may be nothing more than a rotatably mounted waveguide or a ferrite Faraday rotator, and phase shifter 12, in its simplest form, may be a quarter-wavelength plate. The equivalent of a quarter-wavelength plate may also be achieved by periodically loading the waveguide with shunt elements such as irises or screws as explained in the article by Alan J. Simmons entitled, A Method of Producing Broadband Circular Polarization in Square Waveguide, published Jan. 28, 1954, in Naval Research Laboratory Report No. 4286 ASTIA 5458.

A polarization separator 13 having two output ports is connected to the output end of phase shifter 12, one output of the separator going to a load element 14 and the other output of the separator connecting to a standard type of radar receiver designated 15. With respect to polarization separator 13, a rotatably mounted circular waveguide may also be utilized here. Further more, included in the separator, that is, mounted therein, is a one-mode absorber which, as the name implies, absorbs the energy flowing through the separator in one mode and permits the energy in the orthogonal mode to pass through to the radar receiver. A device that may be adapted for use in the present invention as a polarization separator is shown and described in detail on pages 210 through 217 of an article entitled A Broadband Microwave Circulator by Edward A. Ohm, in the IRE Transactions on Microwave Theory and Techniques, dated Oct. 1956.

In considering the operation, the general case will be considered in which linearly polarized information signals and elliptically polarized jamming signals are received by the system. Moreover, for simplicity, square waveguides will be used throughout in explaining the principle of operation. Thus, polarization rotator 11 will be assumed to be a rotatably mounted square waveguide rather than a ferrite Faraday rotator. Similarly, phase shifter 12 and polarization separator 13 will be assumed to include square waveguide housing.

Referring now to FIG. 2 and in particular to FIG. 2a, the desired information signal received by antenna 10 and passed on to rotator element 11 is represented by vector E and for illustrative purposes is shown to be vertically polarized. The multiple polarized interference signal simultaneously received by antenna 10 and rotator 11 is represented by the ellipse designated E,, the major axis of the ellipse being shown making an angle of B with the X or horizontal axis of square waveguide 1 l.

The desired discrimination between the information and jamming signals can be achieved as follows:

a. Mechanically rotating waveguide 11 until the major axis of the jamming ellipse is along the X axis. In other words, waveguide 11 is rotated through an angle of 8 in order to align the major axis of ellipse E, with the X axis of the waveguide. The orientation of the respective polarizations after rotation of waveguide 11 is depicted in FIG. 2b which clearly shows the major axis of ellipse E lying along the X axis of the waveguide and E, forming an angle ,8 with the Y axis of the same waveguide.

b. After rotation of the waveguide 11 toward the directions of polarization as mentioned in (a) above, a 90 phase shift for the vertical modes along the Y axis of the waveguide is introduced which has the effect of converting the information signal from one that IS linearly polarized to one that is elliptically polarized, and quite the opposite. of converting the elliptically polarized jamming signal E, into a linearly polarized signal whose polarization vector E, forms an angle 0 with the X axis. The angle 9 is the angle whose tangent is the ellipticity of the E, signal and is equal to tan (-r), where r is the ellipticity of the ellipse. This relationship is clearly shown in FIG. 2c.

. At this point. the E, and E, signals are again anguiarly displaced relative to the waveguide conductors by mechanically rotating polarization separafor or square waveguide 13 through an angle of 0 until the vector representing the linearly polarized signal E, is aligned with the X axis of the guide. The entire amming signal is now in the X axis mode and may be coupled off into matched load 14 by means of a one-mode coupler device in separator l3. All orthogonal target reflection components along the Y axis of the guide are passed to and received by radar receiver 15. FIG. 2d shows the results of this final rotation, the desired angle 0 being obtained simply by rotating the waveguide for a maximum signal along the X axis during a period when no target reflection is anticipated.

Although it has been indicated above that the jamming signal is ultimately coupled off into matched load 14, it should be noted that it may be of benefit and. therefore, desirable to study the jamming signal instead of dissipating it in the load. Accordingly, other pieces of equipment may be substituted for the load for the purpose of examining the jamming signal. It should be further mentioned that load 14 in which the jamming signal energy is dissipated may also be included in polarization separator 13 as a part thereof, the load in such a case generally constituting a thin wafer-like lossy matenal which is suitably positioned in the separator to dissipate only the unwanted signal mode.

Referring now to FIG. 3. a specific embodiment of the present invention is shown as encompassed by the block diagram of FIG. 1. As shown, the embodiment of FIG. 3 includes a parabolic reflector 20 mounted symmetrically on one end of a circular waveguide 21, the one end of the guide being the input end for received electromagnetic energy. Facing the open input end of waveguide 21 is a splash plate 22 held rigidly in position at the focal point of parabolic reflector 20 by several rods, such as rod 23. it will be recognized immediately by those skilled in the art that the combination of reflector 20, waveguide 21 and splash plate 22 is equivalent to omnipolarization antenna 10 in FIG. 1 for re ceiving signals irrespective of their polarizations.

The operation of the preceding combination will also readily be recognized by those skilled in the art as being one in which signals incident upon parabolic reflector .20 are reflected by it toward its focal point whereat splash plate 22 is located as mentioned previously. The signal energy is further reflected by the splash plate toward waveguide 21, the signals thereafter entering the waveguide at its input end.

The embodiment of FIG. 3 is shown to further include a circular waveguide 24 that is rotatably connected or coupled between circular waveguide 21 and another circular waveguide 25 that is also rotatably mounted. Waveguides 24 and 25 are rotatable individually and this is accomplished by means of separate drive mechamsms 26 and 27. respectively. Each drive mechanism may be a motor whose onoff operation is controlled by a switch. Drive mechanisms 26 and 27 are respectively coupled to waveguides 24 and 25 by means of simple mechanical gearing and linkage arrangements which may be of the type generally designated 28 and 30 in the figure.

MOunted on both waveguides 21 and 24 is a device, generally designated 31, for indicating the number of degrees through which waveguide 24 is rotated relative to waveguide 21. Device 31 comprises a needle or pointer apparatus 32 fixedly mounted on waveguide 21 and a disk 33 fixedly mounted on waveguide 24, disk 33 being graduated in degrees along its periphery. Thus, for example, if needle 32 initially points to 0 on disk 33 and waveguide 24 is thereafter rotated 30, then the final position of the needle will be such that it points to the 30 mark on the disk. It should be mentioned, however, that device 31 is only one way, a very simple way, for providing the desired rotational information and that other techniques are available of a mechanical or electrical nature, or both, for achieving the same result.

Mounted on both waveguides 24 and 25 is a second device, generally designated 34, for indicating the number of degrees through which waveguide 25 is rotated relative to waveguide 24. As before, device 34 includes a needle or pointer mechanism 35 fixedly mounted on waveguide 24 and a disk 36 having degree markings along its periphery fixedly mounted on waveguide 25. The operation of device 34 is exactly the same as that of device 31 and since the operation of the latter has previously been described, no further discussion is deemed necessary as to the operation of the former.

Considering circular waveguide 24 in greater detail, it will be noted from FIG. 3 that a pair of elements 37a and 37b that are preferably triangular shaped are mounted inside the waveguide preferably along a diameter thereof. Elements 37a and 37b produce a 90 phase shift in certain modes of the energy being propagated in waveguides 21 and 24, the particular modes affected in this manner being determined by the orientation of elements 37a and 37b with respect to waveguide 21. It is thus seen that waveguide 24 and elements 37a, 37b correspond to polarization rotator l1 and phase shifter 12, respectively, of FIG. 1. With respect to elements 37a and 37b, these elements may be of any shape and be positioned in waveguide 24 in any manner and, furthermore, may be made of any kind of material to long as the basic requirement is thereby met of providing a 90 phase shift for predetermined modes of the received signal energy. Several polyester materials are available for the manufacture of elements 37a and 37b as well as a material whose well known Trade Name is Rexolite". Rexolite is manufactured by The Rex Corporation, located in West Acton, Massachusetts.

Waveguide 25 is connected at its output end to a microwave adapter 38 whose function is to provide a smooth transition from a circular waveguide to a square or rectangular waveguide for the propagation of the energy therein with a minimum reflective loss. A square or rectangular waveguide-coaxial cable adapter 40 which serves the same purpose as adapter 38 is connected between adapter 38 and a coaxial cable 41. It will be recognized from the earlier discussion and by a comparison between the diagrams of FIGS. 1 and 3 that the combination of waveguide 25 and adapters 38 and 40 is the equivalent of polarization separator 13.

Inside waveguide 25 there is mounted a lossy material to absorb particular modes of the energy propagated through the waveguide'. The lossy material may be in the shape of a rectangular card 42 and may comprise an insulative base, such as glass, upon which a coating of carbon has been deposited. Card 42 is preferably positioned in a plane through the longitudinal axis of waveguide 25 and is rotatable with the waveguide, the particular orientation of the card determining what modes will be attenuated. Obviously card 42 in FIG. 3 corresponds to load 14 in FIG. 1.

In summary, it is again stated that the novel features of the present invention are the acceptance of two orthogonal components of incident energy and the operation on the polarization characteristics of one signal which differ from that of the desired signal in such a way that the desired signal gains emphasis over the undesired signal. The fact that mechanical rotation of the system elements is employed in the embodiments described herein does not preclude other techniques for achieving the same results since any method, including Faraday-effect rotation by means of ferrite materials, may be equally effective.

Having thus described the invention, what is claimed as new is:

1. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and non-linearly polarized, said apparatus comprising: means for simultaneously transforming the unwanted non-linearly polarized signal to an unwanted linearly polarized signal and the wanted linearly polarized signal to a wanted non-linearly polarized signal; and polarization sensitive means for passing only the component of the non-linearly polarized wanted signal whose polarization is substantially orthogonal to that of the unwanted linearly polarized signal.

2. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized, said apparatus comprising: means for simultaneously transforming the unwanted elliptically polarized signal to an unwanted linearly polarized signal and the wanted linearly polarized signal to a wanted elliptically polarized signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal to that of the unwanted linearly polarized signal.

3. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized, said apparatus comprising: means for shifting by the phase of the wanted and unwanted signal components that are polarized perpendicularly to an axis of the ellipse of polarization of theunwanted signal to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal.

4. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized, said apparatus comprising: means for simultaneously transforming the unwanted circularly polarized signal to an unwanted linearly polarized signal and the wanted linearly polarized signal to a wanted elliptically polarized signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal to the polarization of the unwanted signal.

5. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized, said apparatus comprising: means for shifting by 90 the phase of the wanted and unwanted signal components that are polarized perpendicularly to a selected diameter of the circle of polarization of the unwanted signal to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal to the polarization of the linearly polarized unwanted signal.

6. Square waveguide apparatus for separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized, said apparatus comprising: antenna means adapted to receive the linearly and elliptically polarized wanted and unwanted signals, respectively; means for rotating the polarizations of the signals relative to the waveguide to align the major axis of the ellipse of polarization of the unwanted signal with a predetermined axis of the waveguide; phase shifter means for shifting by 90 the phase of the wanted and unwanted signal components that are polarized perpendicularly to said waveguide axis to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; additional means for rotating the polarization of the signals relative to the waveguide to align the polarization of the linearly polarized unwanted signal with said waveguide axis. said additional means including mode-coupler means for passing the component of the elliptically polarized wanted signal that is polan'zed orthogonally to said waveguide axis.

7. The apparatus defined in claim 6 wherein said antenna means includes a waveguide; a parabolic reflector mounted on said waveguide at one end thereof; and a splash plate mounted substantially at the focal point of said parabolic reflector and oriented to face said one end for reflecting toward said one end the wanted and unwanted signal energy incident upon said reflector and reflected therefrom.

8. Square waveguide apparatus for separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized, said apparatus comprising: antenna means for receiving the linearly and circularly polarized wanted and unwanted signals, respectively; means for shifting by 90 the phase of the wanted and unwanted signal components that are polarized perpendicularly to a predetermined axis of the waveguide to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; additional means for rotating the polarizations of die signals relative to the waveguide to align the polarization of the linearly polarized unwanted signal with said waveguide axis, said additional means including modecoupler means for passing the component of the elliptically polarized wanted signal that is polarized orthogonally to said waveguide axis.

9. The apparatus defined in claim 8 wherein said antenna means includes a waveguide; a parabolic reflector mounted on said waveguide at one end thereof; and a splash plate mounted substantially at the focal point of said parabolic reflector and oriented to face said one end for reflecting toward said one end the wanted and unwanted signal energy incident upon said reflector and reflected therefrom.

10. A method of separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized. said method comprising the steps of: transforming the linearly polarized wanted signal to an elliptically polarized signal and simultaneously transforming the elliptically polarized unwanted signal to a linearly polarized signal; and coupling off all components of the wanted and unwanted signals that are polarized in the direction of polarization of said linearly polarized unwanted signal. thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to the direction of polarization of said unwanted signal.

11. A method of separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized. said method comprising the steps of: transforming the linearly polarized wanted signal to an elliptically polarized signal and simultaneously transforming the circularly polarized unwanted signal to a linearly polarized signal; and coupling off all components of the wanted and unwanted signals that are polarized in the direction of polarization of said linearly polarized unwanted signal. thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to the direction of polarization of said unwanted signal.

12. A method of separating wanted from unwanted signals propagated through square waveguide apparatus where said signals are respectively linearly and elliptically polarized. said method comprising the steps of: rotating the polarizations of the signals until the major axis of the ellipse of polarization of the unwanted signal is aligned with a predetermined axis of the waveguide; shifting by the phase of the wanted and unwanted signal components that are polarized perpendicularly to said waveguide axis, thereby to transform the elliptically polarized unwanted signal to a linearly polarized unwanted signal and the linearly polarized wanted signal to an elliptically polarized wanted signal; rotating the polarizations of the signals until the direction of polarization of the linearly polarized unwanted signal is aligned with said waveguide axis; and filtering off all components of the wanted and unwanted signals that are polarized in the direction of said waveguide axis, thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to said waveguide axis.

13. A method of separating wanted from unwanted signals propagated through square waveguide apparatus where said signals are respectively linearly and circularly polarized. said method comprising the steps of: shifting by 90 the phase of the wanted and unwanted signal components that are polarized perpendicularly to a diameter of the circular polarization of the unwanted signal that coincides with a predetermined axis of the waveguide. thereby to transform the linearly polarized wanted signal to an elliptically polarized signal and the circularly polarized unwanted signal to a linearly polarized signal whose direction of polarization is aligned with said waveguide axis; and filtering off all components of the wanted and unwanted signals that are polarized in the direction of said waveguide axis, thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to said waveguide axis.

in m: K i i 

1. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and non-linearly polarized, said apparatus comprising: means for simultaneously transforming the unwanted non-linearly polarized signal to an unwanted linearly polarized signal and the wanted linearly polarized signal to a wanted non-linearly polarized signal; and polarization sensitive means for passing only the component of the non-linearly polarized wanted signal whose polarization is substantially orthogonal to that of the unwanted linearly polarized signal.
 2. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized, said apparatus comprising: means for simultaneously transforming the unwanted elliptically polarized signal to an unwanted linearly polarized signal and the wanted linearly polarized signal to a wanted elliptically polarized signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal to that of the unwanted linearly polarized signal.
 3. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized, said apparatus comprising: means for shifting by 90* the phase of the wanted and unwanted signal components that are polarized perpendicularly to an axis of the ellipse of polarization of theunwanted signal to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal.
 4. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized, said apparatus comprising: means for simultaneously transforming the unwanted circularly polarized signal to an unwanted linearly polarized signal and the wanted linearly polarized signal to a wanted elliptically polarized signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal to the polarization of the unwanted signal.
 5. Apparatus for separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized, said apparatus comprising: means for shifting by 90* the phase of the wanted and unwanted signal components that are polarized perpendicularly to a selected diameter of the circle of polarization of the unwanted signal to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; and polarization sensitive means for passing only the component of the elliptically polarized wanted signal whose polarization is substantially orthogonal to the polarization of the linearly polarized unwanted signal.
 6. Square waveguide apparatus for separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized, said apparatus comprising: antenna means adapted to receive the linearly and elliptically polarized wanted and unwanted signals, respectively; means for rotating the polarizations of the signals relative to the waveguide to align the major axis of the ellipse of polarization of the unwanted signal with a predetermined axis of the waveguide; phase shifter means for shifting by 90* the phase of the wanted and unwanted signal components that are polarized perpendicularly to said waveguide axis to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; additional means for rotating the polarization of the signals relative to the waveguide to align the polarization of the linearly polarized unwanted signal with said waveguide axis, said additional means including mode-coupler means for passing the component of the elliptically polarized wanTed signal that is polarized orthogonally to said waveguide axis.
 7. The apparatus defined in claim 6 wherein said antenna means includes a waveguide; a parabolic reflector mounted on said waveguide at one end thereof; and a splash plate mounted substantially at the focal point of said parabolic reflector and oriented to face said one end for reflecting toward said one end the wanted and unwanted signal energy incident upon said reflector and reflected therefrom.
 8. Square waveguide apparatus for separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized, said apparatus comprising: antenna means for receiving the linearly and circularly polarized wanted and unwanted signals, respectively; means for shifting by 90* the phase of the wanted and unwanted signal components that are polarized perpendicularly to a predetermined axis of the waveguide to produce a linearly polarized unwanted signal and an elliptically polarized wanted signal; additional means for rotating the polarizations of the signals relative to the waveguide to align the polarization of the linearly polarized unwanted signal with said waveguide axis, said additional means including mode-coupler means for passing the component of the elliptically polarized wanted signal that is polarized orthogonally to said waveguide axis.
 9. The apparatus defined in claim 8 wherein said antenna means includes a waveguide; a parabolic reflector mounted on said waveguide at one end thereof; and a splash plate mounted substantially at the focal point of said parabolic reflector and oriented to face said one end for reflecting toward said one end the wanted and unwanted signal energy incident upon said reflector and reflected therefrom.
 10. A method of separating wanted from unwanted signals where said signals are respectively linearly and elliptically polarized, said method comprising the steps of: transforming the linearly polarized wanted signal to an elliptically polarized signal and simultaneously transforming the elliptically polarized unwanted signal to a linearly polarized signal; and coupling off all components of the wanted and unwanted signals that are polarized in the direction of polarization of said linearly polarized unwanted signal, thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to the direction of polarization of said unwanted signal.
 11. A method of separating wanted from unwanted signals where said signals are respectively linearly and circularly polarized, said method comprising the steps of: transforming the linearly polarized wanted signal to an elliptically polarized signal and simultaneously transforming the circularly polarized unwanted signal to a linearly polarized signal; and coupling off all components of the wanted and unwanted signals that are polarized in the direction of polarization of said linearly polarized unwanted signal, thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to the direction of polarization of said unwanted signal.
 12. A method of separating wanted from unwanted signals propagated through square waveguide apparatus where said signals are respectively linearly and elliptically polarized, said method comprising the steps of: rotating the polarizations of the signals until the major axis of the ellipse of polarization of the unwanted signal is aligned with a predetermined axis of the waveguide; shifting by 90* the phase of the wanted and unwanted signal components that are polarized perpendicularly to said waveguide axis, thereby to transform the elliptically polarized unwanted signal to a linearly polarized unwanted signal and the linearly polarized wanted signal to an elliptically polarized wanted signal; rotating the polarizations of the signals until the direction of polarization of the linearly polarized unwanted signal is aligned with said waveguide axis; and filtering off all components of the wanted and unwanted signals that are polarized in the direction of said waveguide axis, thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to said waveguide axis.
 13. A method of separating wanted from unwanted signals propagated through square waveguide apparatus where said signals are respectively linearly and circularly polarized, said method comprising the steps of: shifting by 90* the phase of the wanted and unwanted signal components that are polarized perpendicularly to a diameter of the circular polarization of the unwanted signal that coincides with a predetermined axis of the waveguide, thereby to transform the linearly polarized wanted signal to an elliptically polarized signal and the circularly polarized unwanted signal to a linearly polarized signal whose direction of polarization is aligned with said waveguide axis; and filtering off all components of the wanted and unwanted signals that are polarized in the direction of said waveguide axis, thereby to pass only the component of the elliptically polarized wanted signal that is polarized orthogonally to said waveguide axis. 